Microwave filters



Jan. 7, 1958 M. ARDlTl ETAL 2,819,452

MICROWAVE FILTERS Filed May 8. 1952 2 Sheets-Sheet 1 ELEFANT lNvENToRsMAUR/CE AKD/T EORL-'JES A. DESCHAMPS JACK )122,7 /1d ATTORNEY Jan. 7,1958 M. ARD|T1 ET' AL 2,819,452

MICROWAVE FILTERS :filed May 8. 1952 2 sheets-Sweet 2 Vfl/ 1544 u l I lE /l 3 36 @W1 2 L 32 44x z5/@H12 .2 nl f n im fg \40 l I INVENTORSMAURICE ARDTI GEORES A. DESCHAMPS JACK ELEFANT United States Patent OMICROWAVE FILTERS Maurice Arditi, Clifton, N. J., and Georges A.Deschamps,

New York, and Jack Elefant, Brooklyn, N. Y., assignors to InternationalTelephone and Telegraph Corporation, a corporation of MarylandApplication May 8, 1952, Serial No. 286,7 63

1 Claim. (Cl. S33-73) This invention relates to microwave transmissionsystems and more particularly to microwave filters specially applicableto microwave printed transmission lines and circuitry.

In the copending applications of D. D. Grieg and H. F. Englemann, SerialNo. 234,503, led June 30, 1951, now Patent No. 2,721,312, and M. Arditiand P. Parzen Serial No. 286,764, filed May 8, 1952, now Patent No.2,774,046 a type :of microwave transmission line is disclosedcomprising, in one of its simplest forms, two conductors printed orotherwise disposed in substantially parallel relation on opposite sidesof a strip or layer of dielectric material a small fraction of a quarterwavelength thick. One conductor is made narrower than the other, so thatthe wider planar conductor appears as an infinite conducting surface tothe narrower conductor, thereby insuring the mode of propagation tofmicrowave energy therealong in the TEM mode. The dielectric between thetwo conductors may be of substantially the same Width as the narrowestof the two conductors or wider according to the relationships desired.In our copending application Serial No. 286,761, iiled May 8, 1952, wedisclose filter arrangements utilizing a section of the aforementionedparallel strip type of line and spaced susceptances in the form ofconductor obstacles projecting either partway or all the way across thespace between the paralle'l strip conductors.

An object of this invention is to provide still other microwave filterarrangements which are small, light in Weight, and relatively simple andinexpensive to make, also utilizing a section of the aforementionedparallel type of line.

One of the features of this invention is the manner of providing in aparallel strip type of line spaced susceptances of large value disposedas reflecting shunt impedances to define a resonant section or cavity inthe parallel strip line. The susceptances, broadly speaking, maycomprise the placing of two obstacles or Iother discontinuity structuresin or on one or the other or both of the conductors of the line atspaced points to form a resonant cavity section therebetween. Thesediscontinuities may comprise either a conductor or dielectric obstacle.For example, susceptances may be introduced in the line by placing apiece of conductor cross-wise of the line as `a lumped impedance. Thecross-wise conductor would be in contact with one of the line conductorswith its ends either open or shorted to the other line conductor. Alsosuch obstacles may be printed directly on the strip of dielectric alongwith the line conductors, and if desired, may comprise variations in theshape of the line conductors.

Another feature of the invention is the method and means for tuning theresonant spacing formed by such line conductor configurations. As morefully described hereinafter, this tuning of the resonant spacing orcavity may be accomplished in various ways, including Vernier capacitivescrews, line compressors or stretches, lateral line projections orprotuberances and the size and position of interconnecting portions ofthe line conductor configurations.

ICC

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent by reference tothe following description taken in conjunction with the accompanyingdrawings, wherein:

Fig. 1 is a plan view of `one forni of filter in accordance with theprinciples of this invention;

Fig. 2 is a cross-sectional View taken along line 2-2 of Fig. l; l

Fig. 3 isa plan view of an alternate form of filter;

Fig. 4 is a cross-sectional view taken along line 4 -4 of Fig. 3;

Fig. 5 is a cross-sectional view similar to Fig. 4 showing anothermodification of the invention;

Fig. 6 is a plan view of another embodiment of the invention;

Fig. 7 is a cross-sectional view taken along line 7 7 of Fig. 6;

Fig. 8 is a circle diagram based upon the Smith admittance chart used inexplaining the susceptance characteristics of the plate-'like obstaclesused in the filters of the character disclosed herein;

Fig. 9 is a plan View of a directly coupled filter in accordance withthe principles of this invention;

Fig. 10 is a cross-sectional View taken along line 10- 10 of Fig. 9; and

Figs. l1 and 12 show still other modifications of this invention.

Referring to Figs. 1 and 2, the microwave transmission line shown is ofthe printed circuit type comprising a first or line conductor 1 and asecond or base conductor 2 with a layer 3 of dielectric materialtherebetween. The conductive material may be applied and/ or shaped oretched on a layer of dielectric material, such as polystyrene,polyethylene, quartz, Teflon, fiberglass, or other suitable material ofhigh dielectric quality, in the form of conductive paint or ink, or theconductive material may be chemically deposited, sprayed through astencil, or dusted onto selected prepared surfaces of thedielectric, orby any other of the known printed circuit techniques. The spacing of thetwo conductors is preferably selected a small fraction in the order ofabout 1/10` to about 1/s of a quarterwavelength of the microwavepropagated therealong.

The microwave line of Figs. l and 2 is shown. provided. with spacedobstacles as lumped impedances in the form of short pieces of conductors4, S, 6, and 7 disposed crosswise of the line conductor l. Thesecrosswise conductor obstacles provide two resonant cavity sections oflengths 1, the cavity sections being coupled by quarter wavelengthsections of line. While the crosswise conductors 4 through 7 are shownto be of the same width as the line conductor 1, they may be of otherwidths either wider or narrower as desired, but always a small fractionof a quarter wavelength, depending upon the susceptance value desired.The lengths of the crosswise conductors may also vary depending upon thesusceptance value dev sired. The susceptance value may also be varied byadjusting the position of the crosswise conductor. After the crosswiseconductors have been suitably located, they may be secured to the lineconductor by means of solder 8. After securing the crosswise pieces, itmay be desirable to further tune the susceptances, the cavities idenedthereby or the quarter wavelength spaced therebetween by `some form ofVernier trimming device. Such a trimming device is shown in Figs. l and2 to comprise a piece of small wire as indicated at 9 and 10. Such pieceof wire may be positioned either on the cross pieces 4 to 7 or on theline conductor 1 as may be desired. By adjusting the position of thewire, proper matching may be obtained. The optimum position of trimmingpieces or wire may be determined by use of any suitable measuringtechnique, one satisfactory method of making such measurements beingdisclosed in the pending application of G. A. Deschamps, Serial No.333,164, tiled January 26, 1953. During this adjustment of the wirepieces, it is preferable to have applied thereto a piece of solder whichis maintained soft by a ysoldering iron whereby the Wire is nudged fromone position to another until an optimum reading is obtained whereuponthe solder is permitted to freeze.

When the proper location of the crosswise conductors is obtained, thefilter may be reproduced with reasonable accuracy by photographic andprinted circuit techniques. Such a filter when produced by thesetechniques may have the appearance shown in Figs. 3 and 4. The lineconductor 1a and the cross-conductors 4a through 7a are made integral.While the cross pieces 4a through 7a are shown to extend completelyacross the dielectric 3, it will be clear that they need not be soextended but may fall short of the Width of the dielectric similarly asillustrated in Fig. 1. In order to provide for susceptance trimming, theuse of small pieces of Wire may also be practiced on this form ofprinted filter as well as several other methods. As shown in Figs. 3 and4, one such method comprises conductive posts 11 and 12 disposed in thedielectric 3 in the resonant cavity section of the line. Another methodof varying the length of line between adjacent cross pieces is that ofcompressing or stretching the width of the line conductor. This methodis best employed by providing the line conductor with extended widthwith gradual curvature as indicated at 13. If this width provides ineffect too long a section, that section may be shortened in effect byslicing away edge portions of the line conductor, thus compressing it asindicated between broken lines 14. This lengthening and shortening ofthe line has reference to line wavelength.

The cross pieces 4a through 7a shown in Fig. 3 may be open `or closed attheir ends. In Fig. 4 the cross pieces are shown to be open. In Fig. 5the cross pieces are shown to be closed with respect to the other lineconductor 2a, the line conductor 4a for example being continued by aconductor for connection with conductor 2a. The conductor 2a maycomprise a planar conductor extending the full width of the dielectric 3or it may be of substantially the same width as the line conductor 1a asindicated in Fig. 5. The conductor 2a may also be provided with crosspieces the same as 4a through 7a.

Figs. 6 and 7 show another form of ilter arrangement comprising tworibbon-like conductors 16 and 17 of substantially equal width separatedby a similar region of dielectric 18. The susceptance obstacles oflumped impedance in this iilter comprise strips of conductive material19, 20 and 21, 22 disposed on opposite sides of the line conductors. Thesusceptance values of these obstacles may be adjusted by adjustingrelative positions, one being offset with respect to the other. The endsof these cross pieces may be either open as shown or may be closedsimilarly as indicated in Fig. 5.

Referring to Fig. 8, a Smith admittance chart is shown onto which acircle has been applied corresponding to test data with respect to asusceptance obstacle illustrated at 24 in Fig. 8. The line section `isof the same character illustrated in Fig. l and is provided with likereference characters. The obstacle, however, comprises a crosswise strip25 which is shorted at its ends to conductor 2 as indicated at 26. Itwill be observed that the circle is large thereby indicating that theinsertion loss of this type of obstacle is small. It also shows that theobstacle is substantially symmetrical.

Referring to Figs. 9 and 10, a directly coupled lter is therein showncomprising a line conductor 27 printed on a strip `of dielectric 3 whichin turn is provided with a second conductor 2 on the opposite Vsidethereof. The line conductor 27 is provided with a series of susceptanceobstacles of dilerent lengths, the obstacles 28, 29, 30,

and 31 representing transformer couplingsoflumpedimpedance betweenadjacent resonant sections while the susceptance obstacles 32, 33, and34 determine the susceptance of the resonant sections. These crosswiseobstacles may each be tuned by any one of the tuning or trimming meansherein described but for purposes of illustration the tuning means isshown to be in the form of a capacitive post 35 which may be adjustedwith respect to the opposite line conductor. It should also be observedthat While the second conductor 2 is illustrated as a planar conductorthat it may in fact correspond substantially to the shape of the lineconductor 27 together with the obstacle susceptance projections thereof.

Referring now to Figs. 11 and 12, the iilter may follow various lineconfigurations incorporating susceptance obstacles of lumped impedances.In Fig. 11, for example, the line conductor 36 is shown with obstacleprojections along :one side thereof as indicated Iat 37, the projectionsbeing of a width less than `a quarter wavelength. The printedconfiguration may also incorporate trimming projections `as indicated at38 -and 39. These trimming projections correspond substantially to thesmall pieces of wire 9 and 10 illustrated in Fig. 1. Final trimming ofthe resonant sections and the susceptance obstacles may be accomplishedby cutting away portions of these small projections 38 and 39 until theoptimum lsusceptance is obtained. Should too much be removed, conductivematerial may be added by soldering. In Fig. 12 obstacles may comprisecutouts such as indicated at 40 in the line conductor 41. Thesusceptance values of these recesses in the line 41 are also in thenature of lumped impedances or reflection obstacles, since the width `ofthe slots are a small fraction of a quarter wavelength. These recessesmay be tuned by means of small pieces `of wire las indicated at 42 and43. By adjusting the position of these wires optimum tuning may be had,the Wires being thereafter secured by soldering or other suitablefastening means.

While we have -described above the principles of `our invention inconnection with specific apparatus, it is to be `clearly understood thatthis description is madeonly by way :of example Iand not as a limitationto the scope of our invention as set forth in the objects thereof andIin the `accompanying claim.

We claim:

A microwave filter comprising first and second ribbonlike conductors,means disposing said conductors in dielectrically spaced substantiallyparallel relation a small fraction of a quarter wavelength vapart toprovide a waveguide, `said fir-st conductor being of a width equal to afraction of a quarter wavelength, said second conductor being wider thansaid first conductor to present thereto a planar conducting surface forpropagation `of microwave energy in a mode approximating the TEM mode,said irst conductor having laterally disposed projections extending inoverlying parallel relation to the planar conducting surface of saidsecond conductor, the width of said lateral projections being asmallfraction of a quarter wavelength to present reflecting lump impedancesspaced apart longitudinally of said conductors to form a resonantsection therebetween, and means for adjusting the susceptance value :ofcertain of said lateral projections, said means including a conductivescrew carried by the projection for adjustment into the space betweensaidprojection and said planar conducting surface.

References Cited in the tile of this patent UNITED STATES PATENTS2,411,555 Rogers Nov. 26, 1946I 2,540,488 Mumford Feb. 6,'1951 2,558,748Haeft` July 3, 1951v 2,721,312 Grieg et al. Oct. 18, 1955 OTHERREFERENCES Radio-Electronic Engineering; Sept. 1951, pages 16, 31,S33-84M.

